EFFECT OF FLOW DIRECTION ON AXIAL SOLID DISPERSION IN GAS-SOLIDS COCURRENT UPFLOW AND DOWNFLOW SYSTEMS

Studies on the axial solid mixing mechanisms in gas-solids cocurrent u pflow and downflow circulating fluidized bed systems have revealed tha t, among the many influencing factors, flow direction has the most pro found influence on the axial solids mixing. When the flow is in the di rection of gravity (downflow in the downer), axial solids dispersion i s very small and the flow pattern approaches plug flow; when the flow is against gravity (upflow in the riser), axial solids dispersion is s ignificantly larger and the flow pattern deviates significantly from p lug flow. Solids mixing is found to be mainly due to the dispersion of dispersed particles in the downer while two solids mixing mechanisms co-exist in the riser: the dispersion of dispersed particles and the d ispersion of particle clusters. Dispersion due to dispersed particles is very small in both the riser and the downer, indicating that disper sed particles pass through the system in a near plug flow pattern. Dis persion due to particle clusters in the riser, on the other hand, is v ery significant, contributing to the large axial solids backmixing and the bimodal solids residence time distribution in the riser.